In “sensorless” synchronous electric motors, a measure of a generated electromotive force (EMF) in undriven coils of the motor is used to infer a rotor's position, thereby eliminating the need for Hall effect sensors or a rotary encoder. However, sensorless motors can be difficult to start because no generated EMF is produced when the rotor is stationary. Since measuring generated EMF does not permit a determination of an amount of torque necessary to permit motion in the motor in accordance with a motor condition, initiating motion is generally accomplished by trial and error. That is, the motor is started using any arbitrary phase and skipping to another phase if it the first is found to be wrong. Such trial and error can cause the motor to run briefly backwards, adding even more complexity to the startup sequence.
For the above reasons, prior art sensorless motors are further known to experience added difficulties when the motor is started under unknown load or inertia conditions. These are thus not suited for variable load or inertia conditions. Such difficulties can be caused by driving the motor at an incorrect commutation speed or frequency. An example of such a situation occurs when attempting to start an oil pump over a wide range of oil temperatures.
There is therefore a need to provide improved methods and systems for starting a sensorless motor.